1. Field of the Invention
The present invention relates to an insulated gate type bipolar-transistor (hereafter referred to as "IGBT") integrating an excess voltage protecting function or drain voltage fixing function.
2. Description of the Related Art
Upon turning ON a power switching element employed as an inverting device for a motor circuit or a non-interruption power source circuit, a high voltage can be induced on an inductive load or a floating inductance in the circuit due to abrupt variation of circuit current flowing through the circuit. This high voltage is applied to the power switching element as a spike voltage to cause fatigue or breakage of the power switching element. As a means for protecting the power switching element from excess voltage, a voltage clamp circuit including a constant voltage diode based on the operation principle of avalanche breakdown for turning ON the constant voltage diode before breakdown occurs in the power switching element because of applied spike voltage and thus fixing the level of the spike voltage within a safe operation range, is employed.
In the case that IGBT is employed as the power switching element, a similar means can be applied. In this case, the constant voltage diode is externally connected between the drain and gate the IGBT element. However, it causes an increase in of the assembling cost and also increases the overall circuit dimension. It can form the constant voltage diode integrally with the IGBT element by depositing a polycrystalline silicon layer on the substrates formed thereon the IGBT element to form the constant diode therein. However, this increases the necessary number of photo-masks in the element fabrication process leading to an increase in the production cost. Furthermore, in order to form the constant voltage diode on the surface of the IGBT element, it becomes necessary to make the area for a cell region smaller or increase the chip dimension.
As a solution for this problem, Japanese Unexamined Patent Publication No. 64-81270 proposes a method for integrating the constant voltage diode, in which the constant voltage diode having an operational principle of avalanche breakdown, is incorporated in the IGBT element so that the constant voltage diode is disposed between the drain and the source on the equivalent circuit. This method solves problems, such as narrowing of the chip area of the IGBT element. The proposed construction is shown in FIG. 6.
In FIG. 6, the reference numeral 61 denotes a source electrode, 62 denotes a drain electrode, 63 denotes a gate electrode. The upper layer of a substrate is composed of an insulating gate of DSA construction, p.sup.+ base layer 65, n.sup.+ source layer 66 and forms a p type channel immediately below the gate. On the other hand, observing the vertical structure, the substrate has a npnp four layer structure including n.sup.+ source layer 66, p.sup.+ base layer 65, n.sup.- drain layer 67 and p.sup.+ drain layer 64 between n.sup.+ layer 68 and drain electrode 62.
The p.sup.+ drain layer 64 at the side of the drain electrode 62 is separated into respective small segments and arranged parallel in a honeycomb fashion. This forms a structure, in which a diode with a pn junction is disposed between the collector and the emitter of a pnp transistor parallel thereto. The diode has a portion 69 of n.sup.+ layer projecting into the n.sup.- layer 67 to be positioned closer to the p.sup.+ layer 65 to provide avalanche type diode characteristics and thus protect the IGBT element from excess voltage.
However, in order to incorporate the constant voltage diode, it is essential to provide n.sup.+ layer 68 at the drain side of the substrate. Because of the presence of n.sup.+ layer 68, hole injection from the drain side upon conducting the IGBT is restricted to make the ON voltage higher.
In addition, for fabricating the p.sup.+ layer in the honeycomb configuration at the side of the drain electrode 62 and to extend the part 69 of the n.sup.+ layer 68 into the n.sup.- drain layer 67, the substrate structure becomes complicated and thereby increases costs for fabrication of the wafer and costs for production.